The invention relates to a heat carrier for carrying the heat, received at a heat receiving section of the carrier, to a heat radiating section of the carrier by means of a heating medium.
A typical cooling unit for cooling a heat generating element such as an MPU of a portable personal computer (PC) is disclosed in, for example, Japanese Patent Publication No. 8-303970, which utilizes a heat pipe including therein Wick structure which comprises a multiplicity of fine wires for accommodating a working fluid or heating medium such as water.
In the heat pipe disclosed in the publication, the heating medium evaporates in the heat receiving section (heat generating section) of the heat pipe. The vapor thus generated carries or transports the heat to a heat radiating section, where the heating medium is cooled by liberating the heat and transformed to the liquid state before it is returned to the heat receiving section via the Wick structures, thereby continually carrying heat from the heat receiving section to the heat radiating section, cooling the heat generating section.
Because the heat pipe includes a Wick structure inside it for carrying the working fluid, it cannot be manufactured in a compact form. Hence, if such a heat pipe as mentioned above is employed in an electronic apparatus, the apparatus must become disadvantageously large.
Another type of ecologically safe heating medium in the form of super critical fluid is disclosed in Japanese Patent Publication No. 7-253254.
However, the technology disclosed in this publication is a large system for an air conditioner, and involves a gas pump for circulating a supercritical heating medium between two heat exchangers, one absorbing heat and another radiating heat. Thus, the system is too large to use in electronic apparatuses.
The invention is directed to overcome the foregoing problems by providing a compact heat carrier by using only thin heat pipes but not using conventional thick heat pipes or a pump which makes the size of a heat carrier very large. The invention is also directed to provide a compact heat carrier for use in a compact electronic devices such as a portable PC.
In one aspect, a heat carrier of the invention comprises a closed container having at least one heat receiving section and at least one heat radiating section, and accommodating therein a heating medium, wherein said heating medium is transformed into a supercritical fluid as it is heated in said heat receiving section and gets circulated by convection to carry heat from said heat receiving section to said heat radiating section due to a difference in density based on temperature difference between said heat receiving section and said heat radiating section.
In this arrangement, since the heating medium has a low viscosity in its supercritical state, it requires only a little energy for circulation, thereby enabling manufacture of a compact yet efficient heat carrier.
The closed container may be constructed in the form of a rectangular parallelepiped or a cylinder to facilitate convective circulation of the heating medium within the container.
However, the container is preferably a looped heat pipe connecting the heat receiving section and the heat radiating section, which facilitates smooth flow of the heating medium through the pipe.
The looped heat pipe may be provided with an upward bend which facilitates thermal convection of the heating medium under the influence of the gravitational force.
Covering the entire pipe with a heat insulator, except the heat receiving and radiation sections, will result in enhanced or more efficient circulation of the heating medium, providing better heat transfer from the heat receiving section to the heat radiating section.
The looped heat pipe may be configured to run in zigzags several times between the heat receiving section and the heat radiating section, in place of several pipes connecting the two sections as in conventional heat carrier. This will alleviate burden of filling a heating medium in many pipes.
By installing a multiplicity of down-sized heat radiating sections in the looped heat pipe, an amount of heat to be liberated from each radiating section is reduced, thereby minimizing the dimensions of each radiating section, so that they can be installed in smaller spaces. This is a benefit of the invention especially in down-sizing PCs.
On the other hand, a common heat radiating section can be connected to a multiplicity of heat receiving sections in a looped heat pipe for minimization of space for the heat radiating section and for cutting the cost of the heat carrier.
A portion of the looped heat pipe can be a planar mesh adapted to serve as a heat receiving section, which is suitable for example for a printed circuit board.
In this case, by making the diameter of each pipe smaller than the rest of the heat pipe, heat transfer between the heat receiving section and the heat radiating section may be desirably balanced. This type of heat carrier can be suitably used in a compact light-weight portable PC for example.
A sufficient length of the heat pipe in the heat receiving section may be disposed along the periphery of an object that includes the heat receiving section, thereby allowing efficient heat removal for the apparatus.
Since carbon dioxide advantageously becomes supercritical state at a pressure of 7.3 MPa and a temperature of about 31xc2x0 C., it can be suitably used in cooling an electronic appliance such as a PC which contains elements that generates heat and rises in temperature as high as 80xc2x0 C. if not cooled.